Repair of thin walled structures such as gas turbine blades often requires excavation of defects extending through the structure. Openings/holes that result must be refilled with matching or near matching material to ensure structural integrity. Small openings may be “bridged” with weld metal if surface tension of molten metal to the adjoining substrate is adequate to prevent molten metal drop through due to gravity. However, this technique does not work with relatively larger openings.
Large openings may, in principle, be custom fit with solid preformed repair material which is then welded into the opening with a perimeter weld joint. This practice is tedious, expensive, and requires the manufacturing of a preform after the shape of an opening is determined, or conversely, requires the opening to be formed to accommodate a predetermined preform shape. The inherent restraint of such large preformed material may also induce cracking as the material shrinks from welding. Alternately, large openings may be gradually filled with many smaller weld passes placed around the periphery of the opening until the opening is filled with filler metal. However, this method is also tedious, expensive, and subject to slumping of the weld material as multiple layers are added.
Alternate techniques for bridging large openings by welding include using backing material such as ceramics or matching alloy plates. However, for critical designs such backing plates must be removed. For example, backing plates create stress risers that remain if the backing plate remains, and the backing plate adds mass which may be undesirable in many applications including gas turbine engine blades. Many components are configured such that the back side of the repair is not readily accessible once the cladding layer is formed, and so such backing techniques are not possible.
Another alternate bridging technique involves including partly fused weld filler within the opening and welding over the weld filler to bridge the opening. This practice is known as slugging and often results in weld joints that do not comply with design, drawing, or specification requirements. Consequently, there remains room in the art for an improvement.